Contacts for use in vacuum switch arrangements

ABSTRACT

This invention discloses vacuum interrupter contacts and methods for manufacturing same. The contact compositions and the methods for producing same yield a contact structure which is capable of sustaining an arc at relatively low current conditions so as to prevent the occurrence of the &#39;&#39;&#39;&#39;current chipping&#39;&#39;&#39;&#39; effect. One contact composition which is comprised of spiral wound conductive and vapor cloud forming materials further performs the function of generating an electromagnetic field having a radial component which causes the ends of the arc to move over the contact surfaces to prevent burning of the contact surfaces due to an arc whose ends may remain stationary so long as the arc persists.

United States Patent [191 Bates [451 Jan. 1,1974

[ CONTACTS FOR USE IN VACUUM SWITCH [73] Assignee: l-T-E Imperial Corporation,

Philadelphia, Pa.

22 Filed: July 28,1971

2: App1.No.: 167,024

Related U.S. Application Data [63] Continuation of Ser. No. 750,503, Aug. 6, 1968,

abandoned.

[52] U.S. C1. 200/144, 29/199 [51] Int. Cl. H01h 9/30, HOlh 33/00 [58] Field of Search 29/199; 200/144 B [56] References Cited UNITED STATES PATENTS 2,486,341 10/1949 Stumbock 29/19l.2 3,502,465 5/1970 Nakajima 200/144 B 3,246,979 4/1966 Lafferty 200/144 B 3,497,755 2/1970 Horn 200/144 B 1,386,834 8/1921 Beckert 29/199 1,950,240 3/1934 Hi1genberg..... 200/144 B 3,000,085 9/1961 Green 29/199 3,065,539 11/1962 Hannegan 29/199 3,260,578 7/1962 Stevens 29/191.2 3,372,259 3/1968 Porter 200/144 B 3,469,050 9/1969 Robinson 200/144 B 3,514,850 6/1970 Barber 29/199 3,603,753 9/1971 Frink 200/144 B Primary Examiner-Hyland Bizot Att0rney0strolenk, Faber, Gerb & Soffen [57] ABSTRACT due to an arc whose ends may remain stationary so long as the arc persists.

5 Claims, 7 Drawing Figures PATENTEDJM 1 I874 Agra Kid)

INVENTOR. 4277702 616M755 IE1. Ea-

CONTACTS FOR USE IN VACUUM SWITCH ARRANGEMENTS This is a continuation of application Ser. No. 750,503 filed Aug. 6, 19 68, now'abandoned.

The instant invention relates to vacuum switches, and more particularly to novel contact assemblies for use in vacuum switches which are designed to substantially eliminate the current chopping effect occurring in vacuum switches at low current values which were heretofore unobtainable in conventional vacuum switches; and further providing means for controlling movement of an arc formed between separating vacuum switch contacts at high currents so as to permit high current interruption with minimum metal loss on the vacuum switch contact surfaces. Such control further increases the maximum current that can be interrupted by such vacuum switches and also the number of times that any given current can be successfully interrupted by the vacuum switch apparatus.

Conventional vacuum switches presently in use are typically comprised of first and second contact members substantially vacuum sealed within a suitable housing means. At least one of the contacts is normally secured at one end of the housing means and the second of the two contacts is movably mounted by meansof a flexible member at the second end of the vacuumized housing to permit movement of the contact while at the same time maintaining the vacuumized condition. In the normally closed position, current normally passes from one of the contacts to the other through the mating surfaces of the contacts. During an overload or fault current condition, suitable tripping means is employed for the purpose of separating the contacts. The separation distance is usually relatively small compared to interrupting devices which are exposed to normal atmospheric conditions due to the fact that the vacuumized state within the housing permits satisfactory current interruption with only small separation distances between the cooperating contacts.

Upon the occurrence of an overload or fault current condition, or for any other purpose, the contacts are separated rapidly in the vacuumized medium. The arc formed between the contacts expires much more rapidly than it does when formed between a pair of separating contacts placed in normal atmospheric conditions. The rapid disappearance of the arc is especially true at low current interruptions. This phenomenon causes a current chopping effect causing the current flowing in the circuit to drop to zero value more rapidly than the sinusoidal waveform of the current reaches zero value. At low currents the supply of vapor or the rate at which the vapor is ionized is insufficient to maintain a stable arc. When this condition occurs the arc will extinguish, causing the current to drop abruptly to zero. The abrupt drop in current is called a chop.

This abrupt change in the current through the vacuum switch causes less abrupt but nevertheless rapid changes in the current in the associated circuit and gives rise to high voltages across inductive elements such as transformers, motors and reactors. These voltages can be high enough to damage or break down equipment insulation or circuit insulation. Failure of equipment insulation may be very costly.

The method of alleviating this problem is to reduce the current level at which chopping takes place. If the chopping level can be reduced below the minimum current that will exist in the circuit to which the vacuum interrupter is applied, then chopping will not exist in that installation. At present, a significant number of applications exist where the effects of current chopping are of major and serious concern, hence the need and desire to lower current chopping levels of vacuum interrupters.

One approach presently in use for overcoming the chopping effect is to admix the materials used for the contact with a suitable material which will vaporize in order to form a cloud of charged particles in the region of the arc in order to sustain the arc until the current, following its normal sinusoidal waveform, drops to zero value, at which time the arc will be extinguished.

Some conventional vacuum circuit interrupters employ contact members formed of a suitable conductive material having contact surfaces of a conductive composition comprised of copper and bismuth in predetermined weight ratios. The materials forming the contact surface composition are admixed so that the bismuth is interspersed with the copper in order to act as a vaporizing material during circuit interruption to prevent the current chopping effect. A detailed analysis of the resulting copperbismuth composition has shown that the grain sizes of the copper is of the order of 0.20 inches in diameter. Due to the very low solubility of the bismuth in the copper, most of the bismuth is in the grain boundaries. It is a commonly known fact that the cooperating contacts touch at one or a few very small areas (due to surface imperfections) even though the general shape of the contact surfaces might suggest a broad area of contact. As the contacts separate, during an interruption operation, the first cathode spot or spots are formed at these points of contact. At low currents a single spot would be expected at the last point of contact.

A cathode spot on the contact surface is stated to have a current density of 10 l0 amperes per square centimeter, or 0.645 X 10 to 6.45 X 10 amperes per square inch. At the higher current density value a current of 10 amperes would require a spot having a diameter of approximately 0.0014 inches. The lower current density value requires a cathode spot having a diameter of approximately 0.0044 inches. Experimentation has shown that, due to the large diameter copper grains, extreme difficulty is experienced'with a copper-bismuth composition in the avoidance of current chopping since the possible electrode contact and the initial formation of a cathode spot occurs predominantly on the relatively pure copper grain. The rapid erosion of bismuth which occurs during an arcing condition makes the copper grains high relative to the overall contact surface, causing the copper grains to be the preferred contact point. If, at low currents, i.e., currents below amps, there is only one cathode spot which remains fixed, then the current chopping characteristic would be that of the relatively pure copper grain rather than of the average proportions of the copper and bismuth present in the contact composition.

Thus, the bismuth present in the immediate range of the contact point either burns off so rapidly as to fail to sustain the vapor cloud, or is located a large enough distance away from the contact point so as to fail to sustain the vapor cloud for a period sufficient to allow the sinusoidal waveform to drop to zero value in order to extinguish the are at that time, so that current chopping" will still occur. Summarizing, it can be seen that conventional devices provide successful current interruption at high magnitude currents, but the conventional vacuum switch contact composition is incapable of preventing the current chopping effect for low magnitude currents.

During current interruption in a vacuum switch at any magnitude of current flow, the arc which is formed as the contacts separate causes a burning away or vaporizing of the contact surfaces in the regions of the two contacts which the arc touches. The are, being sustained for some finite period of time, thereby causes damage to the contact surfaces. During the finite pe riod of time in which the arc persists, if the arc remains relatively stationary, substantially severe damage occurs in one specific region on each contact surface, which phenomenon greatly limits the contact life of vacuum switches.

The instant invention is characterized by providing means for forming a contact member for use in vacuum circuit interrupters in which the elements of the mixture (or alloy) used to produce the contact insert are more intimately associated and admixed with one another and are of smaller particle sizes so as to provide a sufficient amount of vapor cloud generating material to sustain an arc until a sinusoidal waveform drops to zero value, thereby overcoming the current chopping effect which would otherwise be present.

The instant invention is comprised of a contact insert formed of copper-and bismuth in which the grain sizes of any one constituent of the mixture has a maximum diameter of 0.010 inches in order to assure the presence of an amount of the vapor producing material in the immediate region of a contact point to be sufficient to sustain an arc until the sinusoidal waveform drops to the zero level.

One method of providing such a composition is comprised of coating a copper tape with a suitable vapor cloud producing material such as, for example, bismuth, antimony or lead, by a vapor coating or plating method. After coating in this manner, the tape is coated with an insulating material and is then wound in a spiral fashion, pressed, and heated. The pressing process acts to remove any voids between adjacent layers of tape and the heating process provides sufficient diffusion of the material. Employing two elements of different resistivity such as, for example, copper and an insulating or low-conductivity material, causes a tendency of the current to flow to the cathode spot via the spiral layers of the resulting contact member. This produces a radial magnetic field component so as to move the spot in order to enhance vaporization of the bismuth as well as preventing excessive deterioration of the contact surface. Such spot movement is desired in order to minimize damaging of the contact surface. In circuit applications wherein the vacuum switch handles currents above the current chopping level, there is no need for providing a low vapor pressure material like bismuth. In fact, since bismuth also has a relatively low melting temperature, its rate of condensation at the time of current zero is somewhat poor, and hence detrimental to the ability of the interrupter to interrupt high currents. Thus, the primary function of the spiral arrangement which generates the radial magnetic field component is that of controlling the rate of vaporization so as to avoid excessive vaporization, spatter of molten metal and excessively heat areas of the cathode. Such excessively heated areas contribute to the failure of the vacuum interrupter to successfully perform an interruption operation.

A second method is comprised of forming a coated powder, or particles, which are compressed together, using a process analogous to vacuum powder metallurgy. By providing grains of sufficiently small diameter, the vapor cloud producing material will be sufficiently uniformly dispersed throughout the powdered material to insure the generation of a vapor cloud capable of sustaining an arc until the sinusoidal waveform falls to the zero level regardless of where the actual cathode spot may be located on the contact surface.

Still another method for producing suitable contacts is to refine the granulated structure of the composition by cold-working and heat treating the resulting composition until grain sizes of suitably small diameters are obtained. In such compositions it is desirable to provide a decreasing amount of vapor cloud generating material with increasing radius of the contact from the cen- -ter of the contact in order to move the spot in the presence of high currents to prevent damaging effects of the anti-current chopping constituents.

As was previously described hereinabove, thin tapes of copper, bismuth and an insulating material may be wrapped together in a spiral fashion to form the contact. The spiral shaped layers of conductive material, i.e., copper, generate a magnetic field component which acts upon the are causing it to move outwardly toward the perimeter of the contact surface. This operation, in moving the arc in this manner, prevents the are from persisting in one specific spot on each of the contact surfaces so as to prevent a substantial region of the contact surface from being deteriorated. By rapidly moving the arc in this manner, the amount of deterioration of the contact surface is significantly diminished due to the fact that the arc will not impinge in any one region of the contact surface for any significant time interval. Similar advantageous results may be obtained merely by wrappiing tapes of insulating material and conductive material, i.e., copper, in spiral fashion in the absence of a tape of bismuth. Since the spiral configuration is still present, the magnetic fields generated to move the arc are likewise present during current interruption, and a significant reduction in deterioration of the contact surfaces is achieved. If desired, the copper and bismuth material can be admixed to form a single tape which may be wound in spiral fashion with the tape of insulating material to form the final spiral configuration of the contact.

The contacts formed by any of the above methods provide a vapor cloud producing material which is dispersed in a sufficiently uniform manner throughout the contact surfaces so as to insure against the occurrence of the chopping effect for a period of time sufficient to allow the sinusoidal current waveform to drop to zero magnitude and thereby to extinguish the arc.

It is therefore one object of the instant invention to provide novel contacts for use in vacuum switch interrupters and the like.

Another object of the instant invention is to provide novel contacts for use in vacuum switch interrupters and the like, which contacts are comprised of a composition having a vapor cloud producing material substantially uniformly dispersed throughout the contacts so as to avoid current chopping by reducing the level at which current chopping occurs to the point where it is no longer a matter of significant application interest.

Another object of the instant invention is to provide novel contacts for use in vacuum switch interrupters and the like, which contacts are comprised of a composition having a vapor cloud producing material substantially uniformly dispersed throughout the contacts and wherein grain size of the materials comprising the composition are controlled to be relatively small in order to avoid the current chopping effects which occur in current interruption of relatively low current arcs, by reducing the level at which current chopping occurs to the point where it is no longer a matter of significant application interest.

Still another object of the instant invention is to provide a novel contact for use in vacuum interrupters and the like, which contacts are comprised of copper and a vapor cloud producing material taken from the group consisting of bismuth, antimony, cadmium, arsenic, manganese and silver and lead, wherein the contact is formed by a method of coating a copper tape with the vapor cloud producing material, winding the tape in a spiral fashion and subjecting the spiral wound assembly to heat and pressure in order to substantially uniformly disperse the vapor cloud producing material within the contact and to control grain sizes of the particles in order to overcome the current chopping effect by reducing the level at which current chopping occurs to the point where it is no longer a matter of significant application interest.

Still another object of the instant invention is to provide a novel contact for use in vacuum interrupters and the like, which contacts are comprised of copper and a vapor cloud producing material taken from the group comprised of bismuth, antimony and lead, wherein the contact is formed by the method of providing coated powder or particles and compressing these in a manner analogous to vacuum powder metallurgy techniques to form a conductor in which the vapor cloud producing material is substantially uniformly dispersed therein.

Another object of the instant invention is to provide a novel contact for use in vacuum interrupters and the like which are comprised of copper and a vapor cloud producing material taken from the group comprised of bismuth, antimony, cadmium, arsenic, manganese, silver and lead, wherein the contact is formed by the method of providing a composition of copper and one of the vapor-forming materials and cold-working and heating the composition in order to break down the particles so as to evenly disperse the vpor cloud producing material throughout the contact to avoid the effect of current chopping occurring during the interruption of low current circuits by reducing the level of current chopping to the point where it is no longer a matter of significant application interest.

Another object of the instant invention is to provide a novel contact for use in vacuum circuit interrupters and the like in which the contact is comprised of a copper-bismuth composition wherein the concentration of bismuth decreases with increasing radial distance from the center of the contact in order to progressively reduce the amount of vapor cloud producing material involved in the arcing process as the magnitude of the current interrupted is increased.

These and other objects of the instant invention will become apparent when reading the accompanying description and drawings in which:

FIG. 1 is a cross-sectional view of a conventional vacuum interrupter.

FIGS. 2a and 2b show the contacts in two different operating positions.

FIG. 2c is a plot of the time-current relationship of the vacuum interrupter.

FIG. 2d shows a detailed view of the engaging surface of one of the contacts with the vacuum interrupter in the closed position.

FIG. 3a is an end view showing one method for forming contacts in accordance with the principles of the instant invention.

FIG. 3b shows an additional step in the method of forming contacts of the composition shown in FIG. 3a.

Referring now to the drawings, FIG. 1 shows a conventional vacuum interrupter l0 comprised of a substantially cylindrical housing 11 formed of an insulating material and being sealed at its upper and lower ends by metallic cover plates 12 and 13, respectively. The cover plates 12 and 13 are sealed by suitable sealing means 14 to the cylindrical housing 11 so as to maintain a vacuumized condition in the interior region of the vacuum interrupter.

The cylindrical housing 11 encloses a pair of cooperating contacts 15 and 16 which are both mechanically and electrically secured to conductive rods 17 and 18, respectively. Conductive rod 17 is secured at its upper end to the metallic cover plate 12 which, in turn, is provided with a suitable terminal 19 which may be employed to connect the circuit interrupter into a circuit which circuit is not shown in FIG. 1 for purposes of brevity.

The lower conductive rod 18 is mounted for reciprocal movement along its longitudinal axis and is coupled at one point 18a along its length to one end of a bellows structure 19 which is air-tightly sealed at one end to conductive rod 18 and at the other end to the cover plate 13 to maintain the vacuumized condition. Cover plate 13 is provided with a suitable aperture 13a to permit the reciprocal movement of the conductive rod 18. The conductive rod 18 may be provided with a suitable terminal 20 for connection into the electrical circuit.

A shield 21 which may preferably be formed of a metallic material substantially surrounds the contacts 15 and 16 to prevent the vaporized material formed during a circuit interruption action from coating the interior surface of cylindrical housing 11 so as to prevent a short-circuit condition between the metallic cover plates 12 and 13.

FIG. 2a shows a detailed view of the contacts 15 and 16 when in the closed position. It can clearly be seen that due to the practical limitations encountered in finishing the surfaces of these contacts that the contact surfaces will not be completely smooth and planar, but will have at least one, and possibly more than one, contact points projecting generally beyond the generally fiat surface of the contacts. Normally, no attempt is made to produce smooth planar surfaces that mate uniformly. Even apparent success along these lines would not yield a solid bar electrical condition. When the contacts separate as a result of an interruption operation, the arc formed during the interruption process will terminate on the surfaces of contacts 15 and 16 at points which are closest to one another such as, for example, the points 15a and 16a. In the interruption of circuits of low magnitude currents and assuming the contacts 15 and 16 to be pure copper, as the contacts separate slightly, the vacuumized condition within the interior of the vacuum interrupter causes rapid dispersal of the small amount of vapor in the arc which soon leads to a condition of insufficient vapor, whereupon the arc is interrupted substantially instantaneously. For example, considering FIG. 2b, let it be assumed that the current waveform of the circuit is a sinusoidal waveform, as shown in FIG. 2c. Let it be assumed that due to a circuit interruption operation the contacts start to separate at time t and that at time t they are separated a very small distance, but a distance which is sufficient to extinguish the arc. Thus, the current will drop substantially instantaneously from a value I to zero current, causing the current chopping effect. Such a large change in the circuit current causes inductive devices which may be associated with the load to produce an extremely high voltage which can either destroy or severely damage such components. It, therefore, becomes necessary to provide some means for sustaining the arc during the time period from t -t at which time the current falls to zero to thereby extinguish the arc.

A conventional manner of sustaining the arc consists of producing a conductive contact formed of copper and bismuth in predetermined weight ratios so that when the arc terminates upon one of the conductive surfaces and makes electrical contact with the bismuth in the contact mixture, the bismuth will be vaporized, forming a vapor cloud 22 of charged particles sufficient to sustain the arc during the time period t -t causing the arc to become extinguished when the current, following its normal sinusoidal waveform falls to zero value. The vapor cloud must be supplied relatively rapidly upon the formation of the arc and there must be a sufficient amount of bismuth present to sustain the arc as the separation between the contacts increases and for a duration of time sufficient to allow the sinusoidal waveform to reach zero current.

The spot diameter of the spot 25, shown in FIG. 2d,

. is of the order of 1 32 1 /16 inch, assuming material having plasticity (as opposed to elasticity) and in the case where the pressure between the contacts is in the range from 100-400 lbs., respectively. Turning to a consideration of FIG. 2d, there is shown therein a greatly enlarged view of the grain configuration appearing on one surface of the contact. The large grains 23 represent the copper grains. The low solubility of the bismuth in the composition causes the bismuth to reside in the grain boundary areas 24. The diameters of the copper grains 23 are of the order of 0.20 inch in conventional contact members. Let it be assumed that the cathode spot contact forms in the region 25, shown in FIG. 2d. It can clearly be seen that its diameter is substantially less than the diameter of the copper grain upon which it is situated so that there is no bismuth available for forming a vapor cloud. Thus, the current chopping effect will in no way be alleviated. If, in terms of the diameter of the cathode spot 25, the bismuth 24 is nearby, then the bismuth will contribute vapor due to its proximity to the spot, in which position it will receive sufficient heat from conduction through metal and from the energy of the positive ions that impact the cathode in greatest density around the cathode spot. Under these conditions, the spot will transfer to a bismuth rich area such as 25'. Although it is desired to have the grains as small as the cathode spot, the desired behavior may be achieved with moderately larger grains.

It is a prime object of the instant invention to provide contacts having a composition in which the maximum grain size of either constituent (i.e., bismuth or copper) is never greater than 0.010 inch and that the bismuth will be dispersed rather evenly throughout the contact surface so as to insure the presence of a sufficient amount of bismuth for generating the vapor cloud to sustain the arc until the current waveform normally falls to zero value.

FIGS. 3a and 3b show one arrangement which may be employed to obtain the desired dispersion of the bismuth material. As shown in FIG. 3a, a thin tape 30 of copper material is coated with a layer 31 of bismuth which is applied by a vapor deposition or plating process. The bismuth may be replaced by or admixed with a material of very low conductivity such as, for example, the material taken from the group comprised of antimony, lead, silver, cadmium and manganese, to form layer 31. A layer of insulating material 32, capable of being compressed and welded into a mass, and capable of being exposed to an are without the formation of gaseous products which would impair the vacuum, may

, be deposited upon the layer 31. The layer 32 does not have to be an insulating layer. All that is required is that it be of low conductivity relative to the compositions of layers 30 and 31. The layer 31 which may be bismuth or one of the other materials mentioned previously, has low conductivity. However layer 31 is very thin and hence, it dies not perform the function performed by layer 32 nor does it greatly change the conductivity of the combination of layers 30-31 from that of copper. Hence, in this example, the material of layer 32 should be of low conductivity relative to copper. Two examples are nichrome and manganese-copper. These materials will also satisfy the other requirements previously mentioned. The tape 30 bearing these deposits is then wound in a spiral fashion, as shown in FIG. 3b, so as to form substantially a disc-shaped assembly. The resulting assembly is then subjected to both heat and pressure, with the pressure being applied upon the spiralshaped assembly to prevent any voids from forming in the assembly and with the heat treatment producing a welding of all of the material in the composition. The resulting substantially disc-shaped contact may then be suitably attached to the conductive rods 17 and 18, shown in FIG. 1, for use in the vacuum interrupter 10.

In use, the spiral configuration in the contact surface will set up a radial component of magnetic field causing the cathode spots such as, for example, the spot 25 or 25', shown in FIG. 2 to experience a force causing the spot to move along the surface of the contact in cases were currents of sufficient magnitudes are being interrupted. This operation prevents a severe melt-out from occurring in that the cathode spot will be moving during the interruption process and thereby prevents a single area from being severely burned due to the presence of the arc.

Another method which may be employed in the production of the contact members is to form powders of the constituent materials. The powders are usually produced by electrolysis or by reduction of an oxide. Pulverizing would appear to be difficult and expensive. Unless there were some prohibiting reason, commercially produced powders can be bought to spccifica tion. Hence, the method of production would not be of interest, unless it created undesirable properties.

The conventional steps of powder metallurgy are:

1 Preparation of the powders.

2 Compacting.

3 Sintering.

4 Coining.

Step (1) involves proportioning the constituents and mixing to obtain uniform composition.

Step (2) involves filling the mold with the proper amount of material, squeezing under a pressure which may be of the order of 50 tons per square inch, relieving the pressure and removing the compact or weak solid from the mold.

Step (3) involves heating sufficiently to permit a bonding (usually by diffusion) at the contact joints of the grains of powder. The temperature would be below the melting point of the main constituent but might be above the melting point of some minor constituent.

Step (4) involves an additional but very heavy pressing operation to increase density (reduce porosity) and produce more accurate dimensions and surface finish.

All of the above steps should be carried out in as high a vacuum as is practicable.

For the embodiment shown in FIG. 3b, the bismuth particles may be so dispersed as to have the highest concentration of bismuth at the center of the contact inserts with the density of bismuth decreasing with increasing radial distance from the center of the contact. This will act to offset the damaging effects of anticurrent chopping constituents for circuit interruption of currents of high magnitude. It should be noted that current flow does not have to be truly along a spiral; it can go cross-lots, provided the spiral causes a sufficient tangential or circumferential component. To provide an idea of how much this component should be, if something of the order of 1 percent of the current could be considered as truly following the spiral, then, at 13,000 amperes, the magnetic effect would be sufficient to spin the are. This supports the idea that crosslots flow is permissible. The objective is such that the spiral distorts the flow so that the total current can be considered as having flowed through some fraction of a turn of the spiral such as one-fifth of a turn. While adequate cathode spot movement may not occur when interrupting low magnitude currents, this will not severely damage the contact surface since severe burnout will not occur due to the low current magnitudes.

Still another method for producing the cotacts is to refine the copper-bismuth composition by coldworking and heat treating the material to break down grain sizes so that they will be of the order of no greater than 0.010 inch in order to produce sufficient vapor clouds during the interruption operation.

It can clearly be seen from the foregoing description that the instant invention provides a novel contact structure for use in vacuum interrupters and the like in which a suitable material is provided for generating a vapor cloud to sustain an arc during interruption so that extinguishment of the arc will occur only when the sinusoidal current waveform falls to zero value and with the vapor cloud producing material and the copper material being so refined and dispersed throughout the contact surface as to generate a vapor cloud of charged particles for a sufficient time period to sustain the arc and hence prevent the current chopping effect.

Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Some modifications which may be employed are: coating plural tapes with bismuth and winding the plural coated tape in spiral fashion; coating the end of the tape which will form the center region of the spiral with bismuth and coating the outer portion with either bismuth and the low conductivity material or with the low-conductivity material along; inserting segments of low conductivity of wedge-shaped configurations in place of the coatings previously mentioned; providing alternate segments of low conductivity interspersed with the copper-bismuth arrangement. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

' The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. In a vacuum switch for establishing and interrupting an electric circuit, said switch having an evacuated chamber; a movable contact-support member operable within said evacuated chamber and a complementary contact-support member in the said chamber; a contact carried by each of said members;

said movable contact support member being movable toward said complementary contact-support member to engage their respective contacts to establish an electric circuit and being movable away from said complementary contact-support member to disengage said contacts and to interrupt said electric circuit; at least one of said contacts having the face thereof,

which engages the other contact, formed of 'a combination of a conductive material and a second material selcted from the group consisting of bismuth, antimony, lead silver and manganese; said conductive material comprising an elongated tape of copper; said tape being spirally wound on an axis perpendicular to the contact face and forming a contact face having a helical line of copper;

said second material being carried on at least one surface of said tape and being spirally wound therewith and being contained, in the contact, between the spiral turns of the copper tape;

said second material also extending to the contact face and forming a helical line adjacent to the helical line of copper;

the contact face thereby having a surface conformation composed of the edge of the copper tape forming a continuous helical line therein and the edge of said second material also forming a helical line therein parallel to and within the helical line of copper;

said second material being capable of vaporizing in the vacuum within the vacuum chamber on separation of the contacts of the vacuum switch to interrupt electrical current.

2. The vacuum switch of claim 1 wherein an additional coating of conductive material having lower conductivity than said second material is carried on the coating of the second material which in turn is carried by said copper tape;

the face of said contact having a surface conformation in which a helical line of said additional material is wound with and parallel to the helical lines of said copper tape and the said second material.

3 The vacuum switch of claim 1 wherein said additional material is formed as a tape wound with the copper tape and adjacent the coating of the second material thereon.

4. The vacuum switch of claim 3 wherein the additional material is manganese copper.

5. The vacuum switch of claim 3 wherein the additional material is nichrome. 

1. In a vacuum switch for establishing and interrupting an electric circuit, said switch having an evacuated chamber; a movable contact-support member operable within said evacuated chamber and a complementary contact-support member in the said chamber; a contact carried by each of said members; said movable contact support member being movable toward said complementary contact-support member to engage their respective contacts to establish an electric circuit and being movable away from said complementary contact-support member to disengage said contacts and to interrupt said electric circuit; at least one of said contacts having the face thereof, which engages the other contact, formed of a combination of a conductive material and a second material selcted from the group consisting of bismuth, antimony, lead silver and manganese; said conductive material comprising an elongated tape of copper; said tape being spirally wound on an axis perpendicular to the contact face and forming a contact face having a helical line of copper; said second material being carried on at least one surface of said tape and being spirally wound therewith and being contained, in the contact, between the spiral turns of the copper tape; said second material also extending to the contact face and forming a helical line adjacent to the helical line of copper; the contact face thereby having a surface conformation composed of the edge of the copper tape forming a continuous helical line therein and the edge of said second material also forming a helical line therein parallel to and within the helical line of copper; said second material being capable of vaporizing in the vacuum within the vacuum chamber on separation of the contacts of the vacuum switch to interrupt electrical current.
 2. The vacuum switch of claim 1 wherein an additional coating of conductive material having lower conductivity than said second maTerial is carried on the coating of the second material which in turn is carried by said copper tape; the face of said contact having a surface conformation in which a helical line of said additional material is wound with and parallel to the helical lines of said copper tape and the said second material.
 3. The vacuum switch of claim 1 wherein said additional material is formed as a tape wound with the copper tape and adjacent the coating of the second material thereon.
 4. The vacuum switch of claim 3 wherein the additional material is manganese copper.
 5. The vacuum switch of claim 3 wherein the additional material is nichrome. 